The clinical significance of right ventricular (RV) infarction has been neglected compared with left ventricular infarction. In recent years, however, the clinical importance of RV function in the treatment of myocardial infarction has been well recognized. We performed prospective radionuclide studies to assess the incidence and prognosis of RV infarction in 50 cases of initial acute myocardial infarction (25 cases of anterior and 25 of inferior infarction). Radionuclide ventriculography was performed within the first two days after onset of symptoms, and repeated one-two weeks and one month after the attack, respectively. RV infarction was diagnosed by the presence of severe RV regional wall motion abnormalities and positive signs of at least one of the following diagnostic signs: ST elevation at V4R in the ECG, positive 99m-technetium pyrophosphate myocardial scintigram at the RV free wall, and positive right heart catheterization findings. Results were as follows: 1. RV infarction was documented in 15 of 25 cases with inferior infarction, but there were no cases in anterior infarction. 2. There were no remarkable changes of RV ejection fraction (EF) in anterior myocardial infarction during one month (41% +/- 8% in acute phase and 43 +/- 8% in four weeks later). However, RVEF was markedly improved from 34 +/- 11% during first two days, to 38 +/- 7% during one-two weeks, and 39 +/- 8% four weeks after the attack, in cases of inferior infarction with RV infarction. Without RV infarction, RVEF in cases of inferior infarction did not show improvement. 3. In 11 of 15 cases with RV infarction, RV regional wall motion abnormalities improved to the normal range, which seemed to contribute to the improvement of RVEF. 4. Hemodynamic findings with Swan-Ganz catheters showed typical findings compatible with RV infarction only in seven of 13 cases with RV infarction. Thus this finding implies that RV failure did not always accompany RV infarction. 5. Coronary arteriography revealed that right coronary arterial lesions proximal to the RV branch were documented in all 10 cases with RV infarction who had coronary arteriography. RV infarction, caused by deranged coronary blood flow at the RV branch of the right coronary artery, showed marked improvement of RVEF during four weeks after the attack in prospective radionuclide studies. This finding was not seen in left ventricular infarction. The pathophysiological mechanism of improvement of RVEF in RV infarction would be the difference of its coronary circulation compared with that of the left ventricle.(ABSTRACT TRUNCATED AT 400 WORDS)